Air-source heat pumps are well known in the art. See, e.g., U.S. Pat. No. 6,615,602 (Wilkinson), which describes a typical air-source heat pump in detail. Air-source heat pumps incorporate a combination of compressors and condensers in a closed-loop system to draw heat energy from the outside air for use in heating interior spaces. They can also be used in reverse to provide for air conditioning of interior spaces. Air-source heat pumps rely on well-known principles of thermodynamics to extract energy from a given volume of air. The maximum energy that can be extracted from a given volume of air by an air-source heat pump is its heating capacity. The heating capacity of an air-source heat pump changes with the temperature of the air from which energy is extracted.
A typical air-source heat pump is arranged with a “high side” and a “low side” configuration, wherein the system refrigerant is at a relatively high pressure and high temperature on the high side and is at a relatively low pressure and low temperature on the low side. Relatively low pressure/low temperature gaseous refrigerant from the low side is introduced into a compressor, which compresses the refrigerant into a high pressure/high temperature gas (compressing a given volume of gas into a smaller volume of gas causes its pressure and temperature to increase). The compressed high pressure/high temperature gas is then forced through a condenser which is in contact with the interior space to be heated; the gas gives up some of its energy in the condenser, thus providing heat to the interior space, and the refrigerant becomes liquefied. The liquid refrigerant is then forced through an expansion device which vaporizes the liquid into a low pressure/low temperature gas. Once the refrigerant has been vaporized into a low pressure/low temperature gas, it is passed through an evaporator which is in contact with the outside air. Heat energy is drawn from the outside air by the refrigerant, which is then introduced to the compressor, repeating the cycle. The portion of the heat pump system between the compressor and the expansion device is the system high side, and the portion of the system from the evaporator back to the compressor is the system low side.
The foregoing is a simplified explanation of the mechanics of how an air-source heat pump works. However, it is sufficient to illustrate a phenomenon of thermodynamics which renders the typical air-source heat pump inefficient in cold climates. As the temperature of the outside air decreases, the expansion device pressurizes less of the refrigerant, resulting in the refrigerant having a lower density (and pressure) for a given volume to achieve a lower boiling point (since the boiling point of the refrigerant must be lower than the temperature of the ambient outside air). As the mass density of the refrigerant decreases eventually the flow of refrigerant will be below the operating capacity of the heat pump. Because air-source heat pumps are designed to handle a specific volume of flow, lowering the amount available lowers the overall heating capacity of the heat pump, because the system high side requires the refrigerant to be of a certain minimum pressure; when the refrigerant pressure is diminished due to decreasing outside air temperatures the compressor must raise the pressure of the refrigerant a greater degree. When the outside temperature becomes cold enough the corresponding pressure differential between the system low side and the system high side becomes too great for the compressor to overcome. To compensate, either compressors with far greater maximum capacity must be used, at great expense and inefficiency, or alternative heat sources must be available when the outside temperature falls too low. Neither of these solutions are practical and thus the use of typical air-source heat pumps is very limited in colder climates, where the need for heat is greatest during those winter months when the outside air is coldest and the resulting heating capacity is lowest.
A solution to the problem of cold climate air-source heat pumps was demonstrated by Shaw in U.S. Pat. No. 5,927,088. Shaw proposed adding a booster compressor in series with and in advance of the primary compressor. The booster compressor is designed to have a greater capacity than the primary compressor, so it can accept lower temperature refrigerant than the primary compressor. The booster compressor also does not need to raise the pressure of the refrigerant to that required of the system high side, but only needs to raise the refrigerant pressure enough to meet the minimum intake pressure requirements of the primary compressor. The use of a booster compressor upstream of the primary compressor thus enables the heat pump to work when cold outside temperatures would reduce the heating capacity of the heat pump to ineffective levels if the primary compressor alone were to be used. As further described by Shaw, the booster compressor would be activated only when the outside air temperature falls below a certain level. When the outside air temperature is sufficiently high, i.e., above the pre-determined activation limit, the booster compressor is bypassed, thus saving the costs of its operation and improving the efficiency of the system as a whole. A system without a booster compressor and having instead a very large capacity primary compressor to accommodate low outside temperatures would have tremendous excess capacity at milder temperatures and thus be quite inefficient to operate.
A known downside to using air-source heat pumps in cold climates is the potential for moisture to freeze onto the evaporator. As ice and frost accumulate onto the evaporator's coils, the transfer of heat becomes less efficient, degrading the entire system's performance. A known solution to this problem is to periodically reverse the flow of refrigerant within the system (as is done when the heat pump is used to cool the interior space), thereby sending heated refrigerant through the evaporator, which then discharges rather than absorbs heat energy, thus defrosting the coils. After allowing the coils to defrost, the flow is restored to permit the refrigerant to absorb heat energy from the outside air. Alternatively, the evaporator coils may be defrosted by the application of heat from an auxiliary heat source, such as an electric heater or a natural gas burner for a predetermined period of time to defrost the coils.
However, these defrost solutions introduce a new problem into the system, namely the potential for the system low side refrigerant to have an initial excess temperature either during or immediately following the defrost cycle. The system low side refrigerant is input into the compressor, as described above; the compressor in turn further increases the temperature of the refrigerant. If the input temperature is initially elevated, the temperature of the refrigerant as it is being compressed may be higher than the limits of the compressor, leading to overheating and potentially damage to or outright failure of the compressor. Where a series of compressors is used, as described by Shaw, the problem is magnified, since the temperature of the refrigerant, elevated as a result of the defrost cycle, is further increased by the booster compressor before the refrigerant is input into the primary compressor.
Another downside to the Shaw design is that the operation of the booster compressor is controlled by the outside air temperature. Thus, when the outside air temperature falls below a predetermined level, the booster compressor is activated. However, it is possible that while the booster compressor is operating the primary compressor may not be operating. This may be for any number of reasons, such as due to an open safety switch, a problem in the control circuitry, power fluctuations, or the like. If the booster were to operate with the primary compressor non-operational, the booster compressor may become damaged and fail. This is because with the primary compressor inoperative the refrigerant will cease to flow through the system. The flow of the refrigerant is used to cool the booster compressor motor and also to circulate oil to lubricate the system (small amounts of oil are carried by the refrigerant). In such circumstances, the booster compressor motor may overheat due to lack of cooling, and may also be damaged due to lack of lubrication. Finally, the accumulated excess refrigerant and oil in the primary compressor reservoir may damage the primary compressor when it becomes operational.
It is an object of this invention to provide a cold climate air-source heat pump which incorporates the efficiencies of the booster compressor design while protecting the overall system from damage from overheating or component shutdown.
It is a further object of this invention to provide a cold climate air-source heat pump which incorporates environmental sensors to detect operating conditions and controllers to initiate and shut down operation of the booster compressor based on inputs from the environmental sensors.
It is a further object of this invention to provide a cold climate air-source heat pump which incorporates an improved defrost means for removing ice from the evaporator coils of the outside evaporator.
Other objects of this invention will be apparent to those skilled in the art from the description and claims which follow.